Here are some of the most common reasons that stocked fish die:
Problems in transport
Fish dying immediately after stocking may have been severely stressed during transport and introduction. The pond’s water temperature should not be greater than 10 degrees more than the water in the distribution unit at stocking. For rainbow trout, the pH of the pond water should not be significantly less than the water the fish have been reared in.
The most frequent and devastating fish die-offs occur after several cloudy days with little wind, which results in too little oxygen in the water. If you see fish that appear to be gulping air at the surface, try to get more water to the pond or adding some sort of aeration.
Pumping water from the bottom of the pond or spraying water across the surface will help aerate the water. Low oxygen levels can also occur in the winter under the ice, particularly if too much snow covers the ice.
If fish start dying within a few days of being stocked and the number of dead fish keeps increasing daily, it is possible the fish were diseased and the disease is killing them. Contact the hatchery that reared the fish for more information.
If there are constantly a few dead fish over a period of time, it is possible that predators such as herons, ducks, or mink are killing fish. Look for tracks and other signs of these species along the shore. There also are some natural parasites that can exist in fish ponds that can mimic the continuous, gradual die-off caused by predators.